Guidelines on Exposure to Electromagnetic Fields from Magnetic Resonance Clinical Systems - Safety Code 26

3. Exposure Guidelines In Other Countries

In the United States, the Food and Drug Administration (FDA) of the Department of Health and Human Services published in 1982 "Guidelines for evaluating electromagnetic risk for trials of clinical NMR^* systems" ⁽¹⁴⁾. The guidelines are directed to sponsors, manufacturers and
researchers of clinical MR devices, and specify the levels of the fields which when exceeded require evaluation in terms of "significant risk". "Significant risk" does not mean that a device is too hazardous for clinical studies. The FDA guideline limits are:

1. static magnetic fields - whole or partial body exposures of 2 T,
2. time-varying magnetic fields - whole or partial body exposures of 3 T/s, and,
3. RF fields - exposure to RF fields that results in a specific absorption rate (SAR) that exceeds 0.4 W/kg as averaged over a whole body, or 2 W/kg as averaged over any one gram of tissue.

In the United Kingdom the National Radiological Protection Board recommended in 1984 that the following conditions be fulfilled during operation of MR clinical imaging equipment⁽¹⁾:

1. The static magnetic field should not exceed 2.5 T to the whole or to a substantial portion of the body for those exposed to the imaging process. Staff operating equipment should not be exposed for prolonged periods of time to more than 0.02 T to the whole body and 0.2 T to arms and hands. For short periods less than 15 min/h these limits are increased to 0.2 T for the whole body and 2 T to arms and hands.
2. The rate of change of magnetic flux density should not exceed 20 T/s for durations of change greater than 10 ms, and for shorter periods the relationship (dB/dt)2 t < 4 should be observed where (dB/dt) is the rms value of the rate of change of the magnetic flux density in T/s and t is the duration of change in s.
3. Exposure to RF fields should not result in a rise in body temperature of more than 1oC (whole body and in any gram of tissue). This may be ensured by limiting the mean specific absorption rate (SAR) to 0.4 W/kg in the whole body, and 4 W/kg in any gram of tissue.

The document also recommends that it might be prudent to exclude from MR imaging pregnant women during the first three months of pregnancy. Persons fitted with cardiac pacemakers and large metallic implants are also subject to special precautions, although they are not excluded from imaging.

In the Federal Republic of Germany, the Federal Health Office published in 1984 "Recommendations for the prevention of health risks caused by magnetic and high-frequency fields in NMR tomography in vivo and NMR spectroscopy" ⁽²²⁾. These recommendations are addressed to physicians using the equipment. The following limits are recommended (for the patient):

1. a static magnetic field of 2 T (whole or partial body); for people wearing cardiac pacemakers exposures to the magnetic flux greater 0.5 mT should be avoided,
2. the time-varying magnetic field should not induce a current density exceeding 3 µA/cm2 for switching times 10 ms and longer, and 30/t_ µ/A/cm2 for shorter switching times, where t (ms) is the switching time,
3. RF exposure should not result in a specific absorption rate (SAR) greater than 1 W/kg, as averaged over the whole body, and 5 W/kg as averaged over any kilogram of tissue, excluding eyes.